Semiconductor lasers have many advantages such as a small size, a low price, low power consumption, and a long life. Due to those advantages, the semiconductor lasers have been widely used in a wide range of fields such as light sources for optical recording, light sources for communication, laser displays, laser printers, and laser pointers. In the field of laser machining, a laser having an optical output exceeding at least 1 W is required. However, none of the semiconductor lasers currently in practical use has reached this output for the reasons described below. Consequently, in the current situation, instead of semiconductor lasers, gas lasers such as carbon dioxide lasers have been used in the field of laser machining.
The reason for the small light output in the semiconductor laser currently in practical use is as follows. In order to increase the light output of the semiconductor laser, it is preferable that the cross-sectional area (emission area) of the laser beam emitted from an element is large. On the other hand, in order to increase the machining accuracy, it is preferable that the cross-sectional area (spot area) of the laser beam irradiated to a workpiece is small. Therefore, ideally, it is desired that the laser beam emitted from the laser source reaches the workpiece as it is without spreading. However, in the semiconductor lasers, as the emission area is increased, laser oscillation occurs in many modes, resulting in disturbed wave front of the laser light. With the disturbed wave front of the laser light, it is difficult to reduce the spot area even if the light is condensed by using an optical system. For this reason, it is difficult for the semiconductor laser currently in practical use to obtain a high light output while reducing the spot area.
Patent Literature 1 discloses a two-dimensional photonic crystal surface emitting laser which is one type of a semiconductor laser. The two-dimensional photonic crystal surface emitting laser is configured to include a two-dimensional photonic crystal and an active layer, where, in the two-dimensional photonic crystal, modified refractive index regions having refractive indexes different from that of a plate-shaped base body are periodically arranged. The modified refractive index region is typically made by holes formed in the base body. In the two-dimensional photonic crystal surface emitting laser, only the light of a predetermined wavelength corresponding to the period of the modified refractive index region among the light generated in the active layer by the injection of the current into the active layer is amplified so that the laser oscillation occurs and emerges as a laser beam in the direction perpendicular to the two-dimensional photonic crystal. Since the two-dimensional photonic crystal surface emitting laser emits light (surface emission) from an area within a certain range in the two-dimensional photonic crystal, the emission area is larger than that of an edge emitting type semiconductor laser, whereby the output power of the light can be easily increased and the divergence angle can be reduced.
Various types of two-dimensional photonic crystals having different shapes and arrangements of modified refractive index regions have been known. The two-dimensional photonic crystal of the two-dimensional photonic crystal surface emitting laser disclosed in Patent Literature 1 has a configuration where, at positions slightly dislocated from each of the modified refractive index regions (main modified refractive index regions) arranged in a square lattice pattern with a period “a”, subsidiary modified refractive index regions of which planar area is smaller than that of the main modified refractive index region are provided. The combination of the main modified refractive index region and the subsidiary modified refractive index region is referred to as a “modified refractive index region pair”.
According to the configuration of Patent Literature 1, in a case where the distance dx in the x direction and the distance dy in the y direction of the subsidiary modified refractive index region from the main modified refractive index region are both 0.25a, in the two-dimensional photonic crystal, among light having the wavelength λ of “a”, the light whose propagation direction is changed by 180° due to reflection at the main modified refractive index region and the light whose propagation direction is changed by 180° due to reflection at the subsidiary modified refractive index region have a difference of 0.5λ in the optical path length, and thus, the two lights are weakened by interference. On the other hand, the light whose propagation direction is changed by 90° in the main modified refractive index region and the light whose propagation direction is changed by 90° in the subsidiary modified refractive index region have a difference of 0.25λ in the optical path length, and thus, the two lights may not be weakened by interference. Since the lights whose propagation direction is changed by 90° in the modified refractive index region pair have a difference of λ in the optical path length with respect to light whose propagation direction is changed in the same direction in the modified refractive index region pair adjacent in the x direction or in the y direction, the lights can be strengthened by interference. The reflection of the light in the direction of 180° causes the localization of light in a partial region in the two-dimensional photonic crystal due to the repetition of this reflection. On the other hand, the change of the propagation direction of the light by 90° contributes to surface emission in a wide range in the two-dimensional photonic crystal. Therefore, by adopting the above configuration, it is possible to strengthen the light in a wide range in the two-dimensional photonic crystal while suppressing the localization of light in a partial region in the two-dimensional photonic crystal, so that it is possible to increase the output power of the laser light.